1. Field of the Invention
The present invention relates to an electrolytic method which allows the desilvering of photographic effluents.
2. Description of the Prior Art
In the photographic development methods, silver ions are released in the processing baths, for example, in the fixing or bleach-fixing baths. The accumulation of these silver ions reduces the treatment efficiency. In order to compensate this degradation, a constant supply of fresh fixer is provided and the used fixing bath is discharged by overflow towards a rinsing tank, then goes down the drain. From an economical and ecological point of view, it is desirable to recover as much silver contained in these photographic effluents as possible, before discharging them in the sewer.
It is known to perform the desilvering of aqueous effluents by numerous processes such as, precipitation for example of Ag.sub.2 S, and oxidation-reduction, for example, on iron wool or by electrolysis.
In order to recover heavy metals such as silver, electrochemical reactions performed, for example, in cells with planar, fixed or mobile electrodes, with or without electrolyte circulation, cells with developed surface electrodes, volume cells, fixed bed or fluidized bed electrodes have been used since a long time. In these cells with [volume electrodes,] the electrolysis is performed by circulating the electrolyte through a conductive particle bed forming the cathode.
These electrodes known as porous electrodes, or volume electrodes, thus allow, due to their high specific surfaces, to treat diluted ionic solutions with a low electrolytic current density and thus to avoid unwanted reactions, particularly when the treated solution must be recycled.
European Patent 37 325 discloses such a device for recovering silver, the yield of which is improved by associating a radial or axial porous electrode to several counter electrodes.
The main drawback of this type of electrolysis is the rapid clogging up of the particle bed forming the volume porous electrode. This clogging up can be particularly rapid in the case of the treatment of some electrolytes, especially the silver ion solutions. In order to overcome this problem, electrolyses of this type were performed through fluidized particle beds in order to prevent the clogging up. However, the electrical conduction inside the bed is then performed under very bad conditions and the electrolytic current densities, much lower than in fixed bed, involve insufficient material transfers, which complicates the use of this method on an industrial scale.
European Patent 329 275 discloses the use of a volume cell which allows to recover the silver contained in photographic effluents ; the Ag content at the cell output is of the order of 100 ppm. This cell with a fluidized bed avoids the clogging up problems, without overcoming the electrical conduction problem. In this case, the silver ion content cannot be less than 100 ppm.
In order to overcome the cell clogging up while maintaining a good electrical conduction, French Patent 2 599 758 (WO 8 707 653) proposes an intermittently stirring of the electrolyte in the volume cells having a radial or colinear field, by pulsation in the electrolyte flow direction.
The silver discharge in the sewer being an environmental problem, it is likely that new more stringent regulations will appear. The prior art discharging, in the sewer, effluents having a silver concentration equal to or more than 100 ppm, it would be desirable to provide a desilvering method which would allow to obtain a silver content, after the desilvering treatment, much less than this value, by a method easy to carry out and economical, while avoiding the clogging up and bad conduction problems.